U.S. patent number 3,960,804 [Application Number 05/509,275] was granted by the patent office on 1976-06-01 for method of making silicone polymer with fillers dispersed therein and using same to make silicone rubber.
Invention is credited to Maurice A. Minuto.
United States Patent |
3,960,804 |
Minuto |
June 1, 1976 |
Method of making silicone polymer with fillers dispersed therein
and using same to make silicone rubber
Abstract
A silicone polymer and fillers are mixed in a high-intensity
mixer until the fillers are dispersed in the silicone polymer, and
the resulting dispersion is then compounded with other compounding
ingredients to obtain a silicone rubber having improved physical
properties.
Inventors: |
Minuto; Maurice A. (Huntington,
NY) |
Family
ID: |
27039662 |
Appl.
No.: |
05/509,275 |
Filed: |
September 25, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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460366 |
Apr 12, 1974 |
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Current U.S.
Class: |
524/588 |
Current CPC
Class: |
C08J
3/203 (20130101); C08J 2383/04 (20130101) |
Current International
Class: |
C08J
3/20 (20060101); C08L 083/04 () |
Field of
Search: |
;260/46.5G,37SB |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jacobs; Lewis T.
Parent Case Text
RELATED APPLICATIONS
This application is a continuation-in-part of application Ser. No.
460,366 filed Apr. 12, 1974, now abandoned.
Claims
What is claimed is:
1. A process for producing a homogeneous mass of silicone polymer
having filler particles dispersed therein, which process comprises
mixing said silicone polymer with said filler at elevated
temperatures between about 0.degree.F. and about 350.degree.F., in
a high-intensity mixer, said mixing being carried out for a period
of time sufficient to disperse said filler particles in said
silicone polymer such that said polymer becomes a continuous phase
with said filler particles dispersed therethrough as the
discontinuous phase.
2. A process as in claim 1 wherein said silicone polymer is an
organopolysiloxane.
3. A process as in claim 2 wherein said organopolysiloxane is a
hologenated dimethylpolysiloxane.
4. A process for producing a homogeneous mass of silicone polymer
having filler particles dispersed therein, which process comprises
mixing said silicone polymer with said filler at elevated
temperatures between about 0.degree.F. and about 350.degree.F., in
a high-intensity mixer, said mixing being carried out for a period
of time sufficient to disperse said filler particles in said
silicone polymer such that said silicone polymer becomes a
continuous phase with said filler particles dispersed therethrough
as the discontinuous phase.
5. A process as in claim 4 wherein said silicone polymer is an
organopolysiloxane.
6. A process as in claim 5 wherein said organopolysiloxane is a
halogenated dimethylpolysiloxane.
7. A process for making silicone rubber compound having improved
physical properties which process comprises (a) mixing, in a
high-intensity mixer, and at elevated temperatures between about
0.degree.F. and 350.degree.F., a silicone polymer with from about
10 to about 100 parts of a reinforcing filler, and from about 15 to
about 500 parts of an extending filler, per part of said silicone
polymer, said mixing being carried out for a period of time
sufficient to disperse said reinforcing filler and said extending
filler in said silicone polymer such that said silicone polymer
becomes a continuous phase with said fillers dispersed therethrough
as discontinuous particulate matters.
8. A process as in claim 7 wherein said silicone polymer is an
organopolysiloxane.
9. A process as in claim 8 wherein said organopolysiloxane is a
halogenated dimethylpolysiloxane.
Description
BACKGROUND OF THE INVENTION
This invention relates to silicone rubber and it is particularly
related to a method of making silicone rubber compounds having
improved physical properties. More specifically, the present
invention is concerned with and contemplates the preparation of a
homogeneous silicone polymer containing fillers dispersed therein,
and the utilization of the resulting dispersion for making silicone
rubber having the same or increased fillers content, while
exhibiting superior physical properties (e.g., tear strength,
tensile strength, elongation and hardness), in a more speedy,
efficient and economical manner than the methods which have
hitherto been known.
THE PRIOR ART
Silicone rubbers and the methods of their manufacture are well
known in the art. Silicone rubbers exhibit exceptional mechanical
and electrical performance and serviceability under extreme
temperature conditions (-100.degree. F. to +600.degree. F.). These
characteristics as well as their inertness, nontoxicity, good
dielectric properties, and resistance to ozone and weatherability
have resulted in their widespread uses in numerous commercial
fields such as in the aerospace, automotive, appliance, electrical
and other industries.
The methods and equipments which are presently employed for
compounding silicone rubber are generally the same as those for
natural rubber or synthetic rubbers. Thus, silicone rubbers are
compounded in conventional equipments such as a doughmixer,
two-roll mill and Banbury mixers. Since these equipments are
ordinarily manufactured for natural or synthetic rubber producers,
appropriate adjustments and modifications of these equipments are
necessary when they are employed for compounding of silicone
rubbers.
The conventional method of compounding silicone rubber on a rubber
mill is as follows; pure silicone rubber polymer (silicone gum) is
first added to a two-roll mill and allowed to band. Fillers (both
reinforcing filler and extending filler) are thereafter added
evenly across the bank and the mixture is blended thoroughly. The
material that falls into the mill pan is scraped and returned to
the batch frequently during compounding. Additives may be
incorporated into the batch during or after the addition of the
fillers, and finally, curing agents are added to complete the
preparation of the silicone rubber compound.
The equipments and techniques which are presently employed for
compounding silicone rubber have certain drawbacks and limitations,
since efficient and practical use of these equipments have imposed
a serious limitation on the amount of fillers which can be
incorporated into the silicone rubber compound. In order to
accomodate higher fillers content during compounding of silicone
rubber, continual adjustments in the equipment must be made which
is cumbersome, uneconomical and often impractical. Increasing the
filler content, particularly the reinforcing filler content, does
not only reduce the cost of the silicone rubber, but also results
in improvements in some of its desirable physical properties.
As it will hereinafter be explained, the use of conventional prior
art methods also requires extended aging of the mixture in order to
insure wetting of the fillers. By "wetting" is meant the adsorption
of the silicone polymer on the fillers (reinforcing fillers and/or
extending fillers).
U.S. Pat. No. 3,824,208 issued recently (July 16, 1974) to Link et
al. describes a method of producing "free-flowing" particles
(powder) of silicone polymers, as well as other polymers, by mixing
the polymer and the fillers in a high-intensity mixer. The
polymeric particles are reduced in size in the mixer and the
fillers are coated on these particulate matters until a
"free-flowing" material is obtained. The underlying objective of
Link et al. is the preparation of "free-flowing" particles, or a
powder, in which the polymer particles are completely coated with
the fillers. Once this has been achieved, the patentees caution
against continued mixing of the fillers with the polymer due to the
formation of "gloubles or small crumps which is not desired." (Col.
9, ls. 17-29)
DETAILED DESCRIPTION OF THE INVENTION
It has now been unexpectedly discovered that when a silicone
polymer (e.g., silicone gum) and fillers are mixed in a
high-intensity mixer for a sufficient period of time, the fillers
become thoroughly dispersed in the silicone polymer thereby
producing a homogeneous mass of silicone polymer having the fillers
thoroughly dispersed therein. It has further been discovered that
the resulting silicone polymer mass, with the fillers dispersed
therein as aforesaid, can be added to a two-roll mill, or charged
directly to an extruder. The remaining compounding ingredients such
as, for example, additives, curing agents and pigments may be added
to the high-intensity mixer during the preparation of the
dispersion of the fillers in the silicone gum, or subsequently to
the two-roll mill or the extruder. Thus all the ingredients
required for the preparation of silicone rubber can be added
directly to the high-intensity mixer therefore obviating the
necessity for the stepwise addition of these ingredients as
required by the prior art techniques.
The homogeneous silicone polymer mass with the fillers dispersed
therein are different from the "free-flowing" particles of silicone
polymer disclosed in Link et al. in that in the practice of the
instant invention the filler particles must be dispersed through a
continuous medium of silicone polymer. Therefore in contrast to the
nature of the "free-flowing" material described in Link et al.
wherein the particles of silicone polymer are completely coated
with the fillers, the practice of this invention requires
dispersing the fillers into the silicone polymer mass, thereby
providing a homogeneous mass of silicone polymer in which the
discrete particulate fillers are surrounded by the silicone
polymer.
In the preparation of silicone rubber, it has been discovered that
unlike the prior art methods in which the fillers are added to the
silicone rubber gum directly in a two-roll mill, a doughmixer or a
Banbury mixer, the pre-dispersion of the fillers (and the remaining
compounding ingredients if desired) into the silicone gum in the
high-intensity mixer facilitates more rapid compounding of the
silicone rubber and the preparation of silicone rubber compounds
having considerably improved physical properties as will
hereinafter be described.
Although it is possible to use pure silicone polymers in the
practice of this invention, for practical and economical reasons it
is generally preferable to employ silicone rubber reinforced gums
(base). In general, the silicone polymers which may be used in the
practice of this invention are disclosed by Maurice Morton in his
book on "Rubber Technology," pages 368-406 (1973), and by Link et
al. in their aforementioned patent. These silicone polymers include
diorganopolysiloxane wherein the organo groups may be monovalent
hydrocarbon, halogenated monovalent hydrocarbon radicals, etc.
The fillers which may be employed in the practice of this invention
are the so-called "reinforcing" fillers and the "extending" fillers
(also known as "semi-reinforcing" fillers). These fillers are also
described in the aforementioned publication and patent, and they
include such materials as Cal-O-Sil (fumed silica), silica aerogels
and, in some instances, carbon black.
The extending fillers which can be employed in the practice of this
invention are also described in the aforementioned publication and
patent and they include such materials as Celite (calcined
diatomaceous silica), Min-U-Sil (ground quartz), titanium oxide,
zinc oxide and iron oxide. The extending fillers are generally
added to achieve an optimum balance between cost, physical
properties and processability of the silicone rubber compound.
Although the use of a processing aid is not necessary in the
practice of this invention, such processing aids, together with
additives curing agents and pigments may be employed either in the
pre-dispersion step, or during compounding. These are conventional
materials with which silicone rubber compounders are well familiar.
For detailed description of the various compounding ingredients
which are suitable for the practice of this invention reference may
be made to the disclosures of the aforementioned publication and
patent, which disclosures are incorporated herein by reference.
As mentioned by Maurice Morton, supra, the selection of the
compounding ingredients and their relative amounts can be varied
depending upon the desired properties of the silicone rubber, and
they may be tailored to suit its ultimate intended applications.
Accordingly it will be appreciated that the amount of fillers which
can be employed in the practice of this invention will vary over a
relatively wide range. Thus, the amount of fillers may generally
vary from about 10 to about 600 parts per 100 parts of the silicone
polymer. However, for practical and economical reasons, the amount
of the reinforcing filler may vary from about 10 to about 100 parts
of the silicone polymer and the amount of the extending filler may
vary from about 15 to about 500 parts per 100 parts of silicone
polymer. Naturally, the maximum amount of filler will vary for
different silicone polymers, in all cases, however, the maximum
amount of the fillers being limited by the amount beyond which it
would be difficult to obtain adequate dispersion of the fillers in
the mass of silicone polymer.
The particles size of the fillers (reinforcing filler and extending
fillers) which are suitable in the practice of this invention are
not per se critical. In general the particles size of the fillers
may vary over a relatively wide range as described in the prior art
such as for example in the aforementioned publication of
Morton.
It must also be mentioned that regardless of the type of fillers
which have been heretofore be employed in the prior art, these
amounts have been limited to no more than 30 to 40 parts of
reinforcing filler per 100 parts of silicone gum. Silicone rubber
compounds having higher fillers content could be produced with
great difficulty, often requiring large amounts of processing aids,
with a consequent poor dispersion and inferior physical properties
of the resulting silicone rubber. However, by the practice of the
invention it is possible to produce silicone rubber compounds
having considerably improved properties even when they contain
considerably higher fillers content.
As it was previously mentioned, even at the levels of the fillers
which have been employed in the prior art techniques, the resulting
compound must be aged in order to insure wetting of the fillers by
the silicone polymer. In general, the silicone gums or base to
which extending fillers have been added must be aged for at least
one day in order to wet the fillers before fabrication. Where
reinforcing fillers are employed, usually at least three days are
required in order to wet the fillers before fabrication.
It has now been also found that in addition to obtaining a
dispersion of the fillers in the silicone rubber by the use of a
high-intensity mixer, it is also possible to achieve various
degrees of wetting of the fillers by the silicones. Thus, by
selecting the optimum time and temperature of the pre-dispersion
step, the wetting of the fillers by the silicone polymer can be
remarkably accelerated thereby obviating the need for extended
aging of the silicone polymer-fillers mixture as it has heretofore
been required in the prior art. In any event, the time required to
achieve a certain degree of wetting in the high-intensity mixer is
considerably less than the time required to achieve a comparable
degree of wetting by the use of the prior art methods.
The speed of the high-intensity mixer, the temperature of the
materials therein and the time required to obtain the substantially
complete and uniform dispersion of the fillers in the silicone
rubber gum can be varied to achieve various degrees of wetting, and
they can be optimized depending upon the type and quantity of the
materials which are employed as well as the desired physical
properties of the silicone rubber compound. Therefore it will be
readily appreciated that, in view of the variety of types of
fillers and silicone polymers which can be employed, and the
differences in their relative amounts, such conditions will vary
over a relatively wide range. In all cases, however, the fillers
and the silicone rubber must be mixed in the high-intensity mixer
for a minimum critical time until the fillers have been
substantially completely dispersed in the silicone polymer. From
visual or photomicroscopic examination of the resulting mixture,
those skilled in the art can readily ascertain when the fillers
have been substantially completely dispersed in the silicone
polymer so as to provide a homogeneous mass of silicone polymer
with fillers dispersed therein. Once such a dispersion has been
obtained, mixing is discontinued and the resulting dispersion can
be added to a two-roll mill, or charged directly to an extruder in
order to prepare the finished silicone rubber compound.
The high-intensity mixer employed herein may be of the usual type
and variety such as for example the Henschel high-intensity mixer.
The speed of rotation (rotor tip speed) of the mixer may vary over
a relatively wide range depending upon the type and relative
amounts of the materials employed. In general it has been found
that the rotor tip speed may vary from about 25 to about 250 feet
per second, preferably from about 50 to about 160 feet per second
satisfactory for most systems, once again, bearing in mind that
this speed may be varied somewhat by the skilled operator during
the mixing of the fillers with the silicone polymer in order to
achieve proper dispersion of the fillers in the silicone
polymer.
As it was previously mentioned, it has been further found that in
addition to obtaining a uniform dispersion of the fillers in the
silicone polymer in the high-intensity mixer, the wetting of the
fillers by the silicone polymer may be improved by achieving an
optimum temperature. Since the temperature of the materials in the
high-intensity mixer rises during the mixing operation due to
frictional heat, a maximum temperature is usually attained after a
finite time when the desired dispersion has been obtained. While at
this temperature, and for most formulations, the fillers are
adequately wetted by the silicone polymer, in other formulations,
it may be desirable to exceed the temperature attained by
frictional heat only. In such instances, the mixer is conveniently
heated by conventional external heating means (e.g., steam coils,
electrical heating) in order to attain the optimum temperature.
Accordingly, it will be appreciated that the optimum temperature
will vary depending, to a great extent, on the type of silicone
polymer, type of fillers, and the relative amounts of these
ingredients. In general, however, this optimum temperature is in
the range of about 0.degree. F. to about 350.degree. F., or even
somewhat higher.
Adequate wetting of the filler by the silicone rubber is highly
desirable from the standpoint of compounding the resulting
dispersion and for the preparation of silicone rubber compounds
having the desired characteristics.
When such large amounts of fillers are added to silicone gums
directly in conventional two-roll mills, in accordance with the
prior art methods, the fillers tend to become highly concentrated
in some parts of the silicone gum forming hard flakes which are
difficult to break up. Accordingly, the mixture is difficult to
compound and the resulting rubber will exhibit inferior physical
properties. On the other hand, when the filler is dispersed in the
silicone gums by the process of this invention, and the resulting
dispersion is then added to the mill, the dispersion bands around
the rollers quickly and uniformly within few minutes, and the
resulting silicone rubber compound exhibits uniform and superior
physical properties. If desired, the resulting dispersion from the
high-intensity mixer can be readily extruded into a comparable
consistency and homogeneity as the milled material prior to
compounding, with the same efficacious results.
The following examples will serve to further illustrate the unique
method of this invention. However, it must be understood that these
examples are not intended to limit this invention, nor the types of
ingredients which can be efficaciously employed therein. In the
following examples, all parts are on weight bases.
EXAMPLE 1
One hundred parts reinforced silicone gum (dimethylpolysiloxane
having added vinyl groups, manufactured under the tradename of Dow
Coring Base 437 by the Dow Coring Corporation, Mildland, Mich.,
Bulletin No. 17-030, January, 1972.) containing approximately 19
weight percent fumed silica was added to a laboratory two-roll mill
having 6 .times. 12 inch rolls and operating at a ratio of 1.25:1,
100.degree. F. and 23 rpm roll speed, without cooling. The silicone
gum readily banded around the roll. Thereafter, a mixture of 100
parts Min-U-Sil (ground quartz) having an average particle size of
5 microns, 35 parts Cab-O-Sil (fumed colloidal silica) having an
average particle size of 0.02 micron and 0.75 part Varox catalyst
(2-5 dimethyl-2, 5-di (t-butyl peroxy) hexane) were added to the
two roll mill. The mixture flaked and then turned into a powder. No
adequate samples could be obtained for testing purposes.
EXAMPLE 2
Using the same ingredients as in Example 1, and in the same
quantities, the silicone gums and the fillers were charged to a
one-gallon Henschel high-intensity mixer rotating at a rotor tip
speed of 157 feet per second (3800 rpm). After six minutes, the
temperature of the material in the mixer rose from room temperature
to 140.degree. F. and the fillers had thoroughly dispersed in the
silicone gums. The resulting dispersion was passed through a
two-inch diameter rubber extruder with a ribbon die and the ribbon
thereafter was added to the same two-roll mill which was employed
in Example 1, and operated at the same conditions. The mixture
readily banded around the rolls and, unlike in the previous
example, it did not flake or turn into a powder.
The resulting band from the two-roll mill was molded into a slab at
320.degree. F. for 20 minutes. Five ASTM size die-C dumbells were
cut from this slab and tested with the following results:
Sample Tensile Strength,.sup..sup.[1.sup.] %
Elongation.sup..sup.[1.sup.] Shore A No. psi
Hardness.sup..sup.[2.sup.] ______________________________________ 1
1210 95 93 2 1195 90 93 3 1200 90 94 4 1190 90 93 5 1215 95 94
______________________________________ .sup..sup.[1.sup.]
ASTM-D-412-68 method was used in this example as well as in the
following examples. .sup..sup.[2.sup.] ASTM-2240-68 method was used
in this examples as well as in the following examples.
EXAMPLE 3
The ingredients employed in this example were the same as in
Example 1.
One hundred parts reinforced silicone gum was added to the two-roll
mill which was employed in Example 1, and which was operated at the
same conditions. The silicone gum readily banded around the roll.
Thereafter, 160 parts Min-U-Sil, 10 parts Cab-O-Sil and 0.8 part
Varox were added to the two-roll mill and were mixed with the
silicone gum. After mixing for 40 minutes, the resulting band was
removed and molded into a slab at 320.degree. F. for 20 minutes.
Five ASTM size die-C dumbells were cut from this slab and their
tensile strengths and elongations were determined as in Example 2.
The results are as follows:
Table 2 ______________________________________ Sample No. Tensile
Strength, % Elongation Shore A psi Hardness
______________________________________ 1 800 130 71 2 730 120 72 3
710 110 74 4 750 125 73 5 810 135 72
______________________________________
EXAMPLE 4
Using the same ingredients as in Example 3, and in the same
quantities, the entire mixture was charged to the high-intensity
mixer employed in Example 2, and were mixed therein at a speed of
3800 rpm. After 51/2 minutes the temperature of the material in the
mixer rose from room temperature to 110.degree. F. The resulting
mixture was then added to the same two-roll mill which was employed
in the previous examples (operating at the same conditions) and the
material readily banded around the roll in two minutes.
The resulting band from the two-roll mill was molded into a slab at
320.degree. F. for 20 minutes. Five ASTM size die-C dumbells were
cut from this slab and the dumbells were tested as in the previous
example, with the following results:
Table 3 ______________________________________ Sample No. Tensile
Strength, % Elongation Shore A psi Hardness
______________________________________ 1 990 140 72 2 1015 155 73 3
1010 150 73 4 1000 150 72 5 1005 150 73
______________________________________
From the foregoing examples it is readily apparent that the present
invention is not only more advantageous when it is desired to
increase the amount of fillers, particularly the reinforcing
filler, in silicone rubber, but it is also more advantageous at
lower levels of fillers content, such as the levels employed in the
prior art methods. Even at such lower levels of fillers, the
resulting silicone rubber compound can be prepared in a
considerably shorter time and without aging of the silicone rubber
gum-filler mixture. In all cases, the resulting silicone rubber
compound exhibits more consistency, higher tensile strength and
higher elongation in comparison with a similar formulation prepared
by the prior art techniques.
The silicone rubber compounds which are produced in accordance with
the method of this invention are generally useful in the same types
of applications as conventional silicone rubbers. Since, however,
the silicone rubber compounds which are made in accordance with the
method of this invention exhibit more consistency, higher tensile
strength, elongation and hardness than the silicone rubbers made by
the prior art methods, they are naturally more useful and desirable
in applications where these improved properties are significant,
such as, for example, in wire coating, molds for casting liquid
plastics and die cast metals, for custom jewelry, ornamental
articles, hardware, furniture, and a host of other
applications.
* * * * *